My invention relates to magnetic core inductive devices such as electrical power transformers and, more particularly, to a construction for a microwave oven power transformer of the high leakage reactance type embodying a coil winding of improved structure. An ancillary invention relates to an improved winding arbor construction useful in production of the coil winding construction.
A transformer is a known inductive type electrical apparatus which converts alternating current at one voltage and frequency typically to another AC voltage functioning by known principles of electromagnetic coupling and transformation between electrical windings on a magnetic core. Those skilled in the transformer art are familiar with the general principles of construction and operation of power transformers, including the contruction of that type of power supply transformer known as a high leakage reactance transformer having a principal application as an element of the power supply in the now familiar microwave oven appliance, as well as electrical insulation requirements thereof. The oven transformer structure in present commercial use includes at least a primary winding coil of enamel covered insulated wire, typically a "precision wound" coil, and a secondary winding coil, also typically a "precision wound" coil, mounted on the central core leg and within the core windows, of conventional E-I laminated magnetic core. The two coils are mounted on the core leg in a side-by-side relationship usually spaced apart a predetermined distance, and magnetic material, known as magnetic shunts, are sandwiched in between the two coils to produce sufficient magnetic leakage reactance between the windings as is desired by the power supply designer using known principles.
Magnetic core inductive devices must have other mechanical and electrical characteristics to be acceptable in commercial practice. Thus a sufficient degree of electrical insulation must be maintained between the electrical wire and the magnetic material and in the case of a microwave oven transformer this degree of insulation is customarily specified as a test breakdown voltage of some given level between the turns of the primary winding and the iron magnetic core material.
In the application of these transformers the iron core is maintained at electrical chassis potential, "ground" or "neutral", as variously termed, and the primary winding is maintained at an AC line voltage, typically 120, 208 or 240 volts in the majority of U.S. communities. To assure the sufficiency of winding to core insulation under normal operation, industry standards require that the primary winding insulation withstand a test voltage, much greater than line voltage, for a short interval. Thus, in present microwave oven transformers, a voltage insulation requirement between the primary winding and the core in the U.S.A. is a test 1,500 volts breakdown minimum applied for a duration of at least five seconds.
Typically, the electrical windings and shunts with the appropriate insulation completely fills the core windows leaving little or no window space. Maintaining the proper insulation would obviously be less difficult if one is permitted to mount the electrical windings in a larger and substantially more expensive transformer core having greater window space. That alternative is obviously not practicable from an economic standpoint.
In present transformer designs with which applicant is familiar, the insulation requirements for the primary winding coil are met in an insulation system design that includes a core insulator, typically a thin wrapper, described as a "core wrapper", on the central core leg over which the primary and secondary windings are installed; a thick spacer of electrically insulative material is inserted or sandwiched in between the side end of the primary coil and the adjacent core yoke leg; an L-shaped section member of electrically insulative paperlike material is located with one flap between the coil side overlying the thick spacer and the second flap thereof anchored between the inner coil periphery and the core wrapper; and a second L-shaped section member is located with one of its flaps in the space between the outer coil surface and the other flap laying over both the first flap and the aforementioned spacer. A still additional thin L-section shaped insulator member is fitted with one flap anchored between the coil's outer peripheral surface and outer core leg and the remaining flap extending down the primary coil's other side end. Thus, the electrical insulation in between the primary coil side and the adjacent yoke leg includes the thick spacer, relatively thick compared to the flaps, and the thicknesses of the two thin flaps of the L-shaped insulators. It is noted also that a principal purpose of one of the first mentioned L-shaped members is to retain the insulative spacer in place during production assembly of the transformer, which prevents the insulative "spacer" from slipping down into a position between the E-shaped lamination and the I-lamination during the manufacturing step of assembling the latter to the former. Likewise, insulation or equivalent spacing is provided between the remaining coil end and any magnetic shunt material adjacent said side.
Applicant's aforedescribed prior art construction satisfies the required electrical voltage breakdown ratings established in the microwave oven field and is produced in large quantities. Because the transformer is produced in large quantities, those skilled in the art recognize that any structural improvement therein which reduces the number of material elements or cost of elements or assembly of the transformer's insulation system, while retaining the desired degree of electrical insulation or improving same, represents a practical advance in the industrial arts and a purposeful assist in conserving natural resources.
To that end a principal object of my invention is to provide a new construction in a precision wound coil for a power transformer or other magnetic core inductor device which satisfies or enhances ancillary voltage insulation requirements, wholly or partially eliminating certain electrically insulative spacers. An ancillary purpose is to provide a suitable transformer construction capable of satisfying the electrical requirements for microwave oven power supply application with reduced material and assembly costs. An additional purpose of the invention is to provide a novel arbor construction useful in producing my transformer invention.
I am aware of a construction for a high leakage reactance transformer presented in U.S. Pat. No. 3,576,508, granted Apr. 27, 1971, in which a portion of the magnetic core is cut away at the corner between the center and yoke legs to enhance electrical insulation characteristics between the electrical coil and the transformer core. I believe that the different structure of my invention hereinafter presented to be a better solution in that it does not require a non-standard transformer core lamination.